An understanding of the genetic mechanisms which influence growth and development of plants, including flowering, provides a means for altering the characteristics of a target plant. Species for which manipulation of growth and/or development characteristics may be advantageous includes all crops, with important examples being the cereals, rice and maize, probably the most agronomically important in warmer climatic zones, and wheat, barley, oats and rye in more temperate climates. Important crops for seed products are oil seed rape and canola, sugar beet, maize, sunflower, soyabean and sorghum. Many crops which are harvested for their roots are, of course, grown annually from seed and the production of seed of any kind is very dependent upon the ability of the plant to flower, to be pollinated and to set seed. In horticulture, control of the timing of growth and development, including flowering, is important. Horticultural plants whose flowering may be controlled include lettuce, endive and vegetable brassicas including cabbage, broccoli and cauliflower, and carnations and geraniums. Dwarf plants on the one hand and over-size, taller plants on the other may be advantageous and/or desirable in various horticultural and agricultural contexts.
Arabidopsis thaliana is a favourite of plant geneticists as a model organism. Because it has a small, well-characterized genome, is relatively easily transformed and regenerated and has a rapid growing cycle, Arabidopsis is an ideal model plant in which to study growth and development and its control.
Many plant growth and developmental processes are regulated by specific members of a family of tetracyclic diterpenoid growth factors known as gibberellins (GA).sup.1. The gai mutation of Arabidopsis confers a dwarf phenotype and a dramatic reduction in GA-responsiveness.sup.2-9. Here we report the molecular cloning of gal via Ds transposon mutagenesis.
The phenotype conferred by the Ds insertion allele confirms that gai is a gain-of-function mutation, and that the wild-type allele (GAI) is dispensable.sup.5,6. GAI encodes a novel polypeptide (GAI) of 532 amino acid residues, of which a 17 amino acid domain is missing in the gai mutant polypeptide. This result is consistent with GAI acting as a plant growth repressor whose activity is antagonized by GA. Though we are not to be bound by any particular theory, gai may repress growth constitutively because it lacks the domain that interacts with the GA signal. Thus according to this model GA regulates plant growth by de-repression.
gai is a dominant, gain-of-function mutation, which confers a dark-green, dwarf phenotype, and interferes with GA reception or subsequent signal-transduction.sup.2-9. Dominant mutations conferring similar phenotypes are known in other plant species, including maize.sup.10-12 and wheat.sup.13. The latter are especially important because they are the basis of the high-yielding, semi-dwarf wheat varieties of the `green revolution`.sup.14. The increased yield of these varieties is due to an increased grain production per ear, and superior straw strength. The shorter, stronger straw greatly reduces the losses resulting from lodging, that is flattening of standing wheat plants by rain/wind. We set out to clone gal from Arabidopsis because of its importance to the understanding of GA signal-transduction, and because of the potential for use of GA-insensitivity in the development of wheat and other crops such as oil-seed rape and rice which may show improvement as great as that already seen in wheat.